1. Field of the Invention
The invention relates generally to surface treatment of threaded connections for oil and gas well casing and tubing with metal-to-metal seals. More particularly, the invention relates to setting a ratio of phosphophyllite (Zn2Fe(PO4)2·4H2O) and hopeite (Zn3(PO4)2·4H2O) in a zinc phosphate treatment of a threaded connection to provide improved resistance to galling.
2. Background Art
Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be emplaced in a wellbore to stabilize a formation or to protect a formation against elevated wellbore pressures (e.g., wellbore pressures that exceed a formation pressure).
Casing joints may be coupled in an end-to-end manner by threaded connections designed so as to form a seal between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be a metal-to-metal seal formed proximate the connection. Examples of such metal-to-metal seals are taught in U.S. Pat. No. 5,423,579 issued to Blose, et al. and Pat. No. 2,893,759 issued to Blose, both of which are assigned to the assignee of the present invention.
Metal-to-metal seals used in threaded connections for relatively thin-walled oilfield tubulars may have shallow seal angles of less than 10 degrees, measured from the centerline of the threaded connection, in order to use the thin walls of the pipe most effectively.
Large diameter oilfield tubulars, because of their relatively high D/t ratio (the ratio of the nominal outer diameter of the pipe to the nominal wall thickness of the pipe) can suffer from ovality or other eccentricities due to manufacturing variables or damage from improper handling. When metal-to-metal seals are used on threaded connections for large diameter casing, the thread designer is often obligated to increase the interference between the seal surfaces at make-up (that is, when the connection is screwed together) to ensure that any eccentricity of the pipe does not compromise the seal. Seal interference is typically measured in thousandths of an inch of diametral interference per inch of nominal pipe diameter. Seal interferences may fall within the range of 0.0025″ to 0.0045″ per inch of nominal pipe diameter. For example, a metal-to-metal seal on 16″ nominal diameter pipe may have an interference at make-up of 0.040″ to 0.072″ on diameter.
As used herein, large diameter oilfield tubulars generally means pipes having a nominal diameter of greater than or equal to 9.375″, although the benefits of the current invention are not necessarily limited to pipes of this dimension.
Furthermore, it is now commonly required by companies which drill oil and gas wells that the threaded connections on large diameter casing be capable of multiple make-and-break cycles, that is, the connections must be made-up (screwed together) and broken-out (screwed apart) multiple times without deleterious effects such as galling of the metal-to-metal seal surfaces.
Galling is a well known phenomenon to those having ordinary skill in the art. Galling may be thought of as a deleterious loss of lubrication. Galling is often caused by high contract stresses over long surfaces. Therefore, galling is particularly significant for oilfield tubulars having shallow contact angles (shallow contact angles, of course, lead to long contact stresses). Further, it is also common in large diameter tubulars, because of the higher contact stress associated with the larger diameters.
The requirement of being able to make up the connections numerous times is intended to ensure that if a problem is encountered while running the casing into the well, such as an obstruction in the well bore, that the casing can be removed from the well bore while the problem is rectified, and the casing can then be re-installed. A common version of this make-and-break requirement is that the threaded connection must be capable of being made-up three times and broken-out twice, which simulates two problem runs followed by a completed run of casing into the wellbore.
When threaded connections for oilfield connections are made-up, conventionally they are lubricated by “thread compound”, commonly called “pipe dope”. Thread compound is most commonly a grease-based compound with entrained particles of lead, copper, or tin, or graphite or their oxides or sulfides, or similar inorganic materials, or alternatively PTFE or other synthetic materials. These particles typically may range in size from 1 micron to about 75 microns, but may sometimes be greater than 100 microns in size.
U.S. Pat. No. 2,543,741 (Zweifel) for example, teaches a lubricating composition for threaded joints which contains flake copper, powdered lead, and graphite. The copper flakes in the composition are very small, and ordinarily will be of substantially uniform thickness, within the range of 3.9×10−5 to 19.5×10−5 inches (or about 1 micron to 5 microns). It is taught that the copper flake should not exceed, in its greatest dimensions, about 76 microns.
U.S. Pat. No. 2,754,266 (Stegemeier, et al.) teaches an electrically conductive thread compound containing fine particles of metals, metal oxides, or metal sulfides, with a maximum particle size of less than about 0.002″ (about 50.8 microns), and preferably less than 0.001″ (about 25.4 microns).
U.S. Pat. No. 3,423,315 (McCarthy, et al.) teaches a pipe thread lubricant containing powdered lead particles where about 63 percent of the lead particles will pass a 325 mesh sieve (that is, are less than 44 microns in size) and where another 14 percent of the lead particles will pass a 200 mesh sieve (that is, are less than 74 microns in size).
U.S. Pat. No. 3,935,114 (Donaho) teaches a low-wear grease for journal bearings on oilfield drilling bits which contains molybdenum disulfide having a range of particle sizes, including some fine and some coarse particles, with 100 percent of the particles passing though a 100 mesh sieve (that is, are less than 149 microns), and 85 percent of the particles passing through a 325 mesh sieve (that is, are less than 44 microns).
When threaded connections on large-diameter casing have shallow-angle metal-to-metal seals (that is, seals with seal angles less than 10 degrees) used in combination with high seal interferences, the metal-to-metal seal surfaces may be in contact for several rotations of the connections during make-up.
Furthermore, if the thread pitch (conventionally denominated as threads per inch) is high, the metal-to-metal seal surfaces may be in contact for more rotations of the connections during make-up than if the thread pitch is lower. Thread pitch for threaded connections for oilfield tubulars are typically in the range of 2 threads per inch to 6 threads per inch.
Overall, this combination of variables can yield a situation in which the two steel surfaces of the metal-to-metal seals are in sliding contact, under stresses normal to the contact surfaces, for a considerable helical distance. This can mean that under these conditions, the metal-to-metal seal surfaces are highly prone to galling on repeated make-break cycles.
Conventionally, phosphate coatings may be applied to threaded connections on oilfield tubulars to reduce corrosion during storage and to improve the retention of the thread lubricant (“pipe dope”) during make-up. Various types of phosphate coatings are used for this purpose. For example, drill pipe threads, which can see hundreds of make-and-break cycles during their life-time, are commonly coated with a heavy manganese phosphate treatment. Manganese phosphate coatings are typically thicker, harder, have better thermal stability, and resist burnishing better than zinc phosphate coatings.
However, manganese phosphate treatments can be much more expensive and time-consuming to apply than zinc phosphate treatments.
Zinc phosphate coatings have been widely used industrially as an undercoating for paint in order to improve the adhesion of the paint and to improve corrosion resistance. Zinc phosphate coatings on a steel substrate are typically composed of two hydrated minerals: hopeite (zinc phosphate, Zn3(PO4)2·4H2O), and phosphophyllite (zinc iron phosphate, Zn2Fe(PO4)2·4H2O). Hopeite crystals are generally orthorhombic in form, with a Moh's hardness of about 4. Phosphophyllite crystals are generally monoclinic in form, with a Moh's hardness of about 3.
It is known that (a) the weight of the phosphate coating, typically measured in grams per square meter (gm/m2), and (b) the proportions of phosphophyllite and hopeite in the coatings, have a profound effect on the adhesion of subsequent coatings (such as paint), and on corrosion resistance, and that these are important characteristics that determine the efficiency of a phosphate coating.
The proportions of phosphophyllite and hopeite in a zinc phosphate coating are often expressed as ratio called the “Phosphophyllite Ratio”, or “P-Ratio”, which is expressed as:P-Ratio=lp/lp+lh  (Eq. 1)where lh represents the X-Ray diffraction intensity from the surface of hopeite and lp represents the X-Ray diffraction intensity from the surface of phosphophyllite. The P-Ratio is widely recognized as a characterizing value for the zinc phosphate-type films used as paint-base coatings on iron and steel
Traditionally, the iron required to form phosphophyllite in a zinc phosphating system is obtained by dissolution of a steel substrate. In some modern zinc phosphating systems, available iron may also be added to the phosphating solution. In the case of “tri-cation” phosphating systems, nickel and manganese may also be added to the phosphating bath.
For the purposes of measuring the P-Ratio with reference to the present invention, it is assumed, as is the current practice in the art, that “phosphophyllite” in the P-Ratio encompasses not only zinc iron phosphate, but also any of its analogues in which manganese, nickel, cobalt, calcium, magnesium, copper, and/or similar cations that were dissolved in the phosphating solution can replace some or all of the iron in the phosphophyllite.
The standard method for determining the P-ratio is by X-ray diffraction; however, it can also be determined experimentally by a number of methods, including, for example, a reagent method taught in U.S. Pat. No. 4,544,639 (Faust).
When a zinc phosphate coating is used as an undercoat on steel for a painted surface (for example, in automotive or appliance applications), it is well-known in the art that the resistance of the painted surface to salt-spray and scab corrosion is greatly improved by maintaining a very high P-Ratio in the zinc phosphate undercoat. That is, it is desirable to reduce the percentage of hopeite in the zinc phosphate coating used for paint undercoating.
As taught in U.S. Pat. No. 6,179,934 (Kawakami, et al.), Col. 2, Line 41, P-Ratio values of 0.8 to 1.0 are considered to provide good conversion coatings for paint-based coatings.
U.S. Pat. No. 6,612,415 (Yamane) discloses that the most preferable P-Ratio for phosphate coating of a steel disc-brake shoe to effect good bonding between the backing plate and the friction material is 0.8 to 1.0.
U.S. Pat. No. 4,510,209 (Hada, et al.) teaches a two layer-coated steel material with a base coat of zinc or zinc-based alloy (essentially galvanized steel), and a surface coating layer of zinc phosphate, to improve the performance of later over coatings such as paint. The '209 patent further teaches that when a conventional zinc phosphate treatment is applied to zinc or zinc-based alloy-plated steel material, the resultant phosphate coating film mainly comprises a hopeite-type zinc phosphate (Zn3(PO4)2·4H2O) in the form of needle-like crystals.
It is known in the art that a zinc phosphate treatment applied to pure zinc will yield 100% Hopeite in the absence of any free iron in the solution. As will be discussed later, a higher Hopeite percentage in a zinc phosphate coating on a threaded connection for oilfield tubulars would be highly desirable, but the added expense of the required zinc base coat required by the process taught by the '209 patent makes it uneconomical for oilfield threaded connections.
Nevertheless, as shown in FIG. 1 (FIG. 3 of the '209 patent), the '209 patent teaches that there are three distinct “zones” of crystal formation in zinc phosphate coatings on galvanized steel, depending on the concentration of zinc in the coating, as follows: (a) When the content of zinc is between 2% and 40% by weight, the resulting phosphate layer consists mainly of zinc iron phosphate, or phosphophyllite, in the form of fine particle-shaped dense crystals. As shown in FIG. 1, Curve II, this zone is labeled “Grain Shaped Crystals”. (b) When the content of zinc is in the range from about 40% to about 60% by weight, the phosphate film layer consists of a mixture of the fine particle-shaped phosphophyllite crystals and coarse needle-shaped hopeite crystals. As shown in FIG. 1, Curve II, this zone is labeled “Mixture of Needle-Shaped Crystals and Grain Shaped Crystals.” (c) When the content of zinc exceeds about 60% by weight, the phosphate layer consists mainly of the hopeite crystals. As shown in FIG. 1, Curve II, this zone is labeled “Needle-Shaped Crystals.” This chart demonstrates clearly the continuum of the effect on the crystal morphology (including both crystal size and shape) of a zinc phosphate coating as the ratio of zinc to iron in the coating changes.
In general, hopeite crystals tend to grow much larger (reportedly up to 20-50 microns in length) than phosphophyllite crystals, and in random orientations, including some crystals growing vertically, or normal to the plane of the substrate. The random pattern of hopeite crystal growth results in relatively large interstices between the crystals. The combination of relatively large crystal size, random (and sometimes vertical) crystal growth, and large interstices has been shown to adversely affect the adhesion of overcoats (such as paints) to a hopeite-rich zinc phosphate layer.
What is still needed, however, are methods for improving the performance of a zinc phosphate coating to meet a requirement for multiple make-and-break cycles rather than to use a manganese phosphate system, or other expensive phosphating system, for threaded connections on oilfield casing.